An optoelectronic switching device having a function in which the internal state is changed by a light information signal of a small energy, and the light information is stored is an indispensable device for a parallel information processing system, an optical switching system, an optical measurement system, etc. Such an optoelectronic switching device is described on pages 596 to 600 of "Journal of Applied Physics, 59(2), 15, Jan. 1986." This optoelectronic switching device is a pnpn thyristor structure, and comprises an anode layer of p-AlGaAs, a cathode layer of n-AlGaAs, an n-gate layer of n-GaAs, and a p-gate layer of p-GaAs, wherein bandgap energies of the n- and p-gate layers are narrower than those of the anode and cathode layers, and the n- and p-gate layers are sandwiched by the anode and cathode layers.
The pnpn thyristor device is changed from the high impedance state to the low impedance state in accordance with its turning-on. In this low impedance state, carriers are mainly injected into the n-gate layer and confined therein, because the gate layers are formed by semiconductors having the bandgap energies narrower than those of the anode and cathode layers. As a result, light is emitted from the pnpn thyristor device. Although the p-gate layer is of a high concentration and a thin film in the pnpn thyristor device to improve the light emitting efficiency and the light emitting sensitivity, this pnpn thyristor device is basically a pnpn semiconductor device.
Such a pnpn semiconductor device is turned on to shift from the high impedance state to the low impedance state in accordance with an applied voltage which is more than a switching voltage Vs. This switching voltage Vs shifts in a direction of a low voltage in accordance with an input light incident to the pnpn semiconductor device. The shifted switching voltage is indicated by Vss hereinafter. Once the pnpn semiconductor device is turned on in accordance with the input light incident thereto and the shifted switching voltage Vss applied thereto, it remains turned on even after the shutting-off of the input light. The low impedance state is held by applying a holding voltage lower than the shifted switching voltage to the pnpn semiconductor device, and reset by applying a resetting voltage lower than the holding voltage to the pnpn semiconductor device. Therefore, the pnpn semiconductor device can be used for an optical memory device.
As understood from the above, this pnpn semiconductor device is turned on by an incident light having a predetermined light amount in the state that a predetermined switching voltage Vss is applied to the pnpn semiconductor device. Therefore, a logic circuit such as "AND", "OR", etc. is realized by controlling the light amount of the incident light, so that various logic calculations are carried out by combining the logic circuits. In fabricating an AND circuit, for instance, two pulse light signals A and B are supplied to the pnpn semiconductor device simultaneously. In this case, a total energy of the two pulse light signals A and B is a level by which the pnpn semiconductor device is turned on in the state that a predetermined switching voltage Vss is applied thereto, although the pnpn semiconductor device is not turned on due to the shortage of the light energy, where at least one of the two pulse light signals A and B is not applied thereto. The state of the low impedance in which the pnpn semiconductor device is turned on by receiving the two pulse light signals A and B is held therein by applying a predetermined voltage slightly higher than a holding voltage Vh and lower than the switching voltage Vss to the pnpn semiconductor device. Then, the switching voltage Vss which is a reading-out voltage is applied to the pnpn semiconductor device, so that a pulse light signal C is emitted from the pnpn semiconductor device in accordance with the turning-on state. On the other hand, the pulse light signal C is not obtained from the pnpn semiconductor device, where at least one of the two pulse light signals A and B is not applied to the pnpn semiconductor device, so that the low impedance state is not obtained therein. As a result, the following truth table which corresponds to a calculation in the AND circuit is obtained.
______________________________________ INPUT OUTPUT A B C ______________________________________ 1 1 1 1 0 0 0 1 0 0 0 0 ______________________________________
However, a conventional method for driving an optoelectronic switching device has a disadvantage in that plural light sources are necessary to provide a logic circuit, as described above in the case where the pnpn semiconductor device is turned on and off to realize the AND circuit. Therefore, where a predetermined number of the pnpn semiconductor devices are integrated in a predetermined area to carry out an optical parallel calculation, the integration circuit density is hard to be increased in the areal view point. A further disadvantage is that the optical alignment of optical systems for supplying plural input light signals to the pnpn semiconductor devices and receiving plural output light signals therefrom is difficult, and the number of power supplies is inevitably increased to result in the difficulty of controlling the timings of the turning on and off.